The present invention relates to a process for preparing 1,5-pentanediols by single-stage reaction of alkoxydihydropyrans with water and hydrogen in the presence of a catalyst and also to the use of the catalyst in this process.
1,5-pentanediol is an important intermediate for the production of polyesters, polyurethanes and heterocyclic compounds such as 1-methylpiperidine which are used as intermediates in the production of drugs and crop protection agents.
Pentanediol is produced industrially on a large scale, as described in Ullmann""s Encyclopedia of Industrial Chemistry, 5th Edition, Vol. Al, p. 307, by catalytic hydrogenation of glutaric acid, which is obtained industrially as a coproduct in the production of adipic acid, or its esters. In the catalytic hydrogenation of adipic acid/glutaric acid mixtures, pentanediol is obtained in a proportion of only about 10 mol%. Furthermore, the isolation of very pure pentanediol from this mixture is difficult because of the very similar boiling points of pentanediol and hexanediol.
A further process for preparing pentanediol starts from glutaraldehyde which is obtained by acid hydrolysis of alkoxydihydropyrans. Processes for the hydrogenation of dialdehydes are described in DE-A 4 414 274; in these processes, it is also possible to use the hemiacetals or acetals of the dialdehydes. As catalyst, use is made of a monolithic catalyst comprising a noble metal on a metal support coated with aluminum/silicon oxide. A disadvantage of these processes is that a two-stage reaction starting from alkoxydihydropyrans, namely acid hydrolysis and subsequent hydrogenation, is necessary.
There have therefore been attempts to convert alkoxydihydropyran directly into pentanediol in a single-stage process. DE-A 1 003 704 and U.S. Pat. No. 2,546,019 describe processes in which pentanediol is obtained by single-stage reaction of alkoxydihydropyran with water and hydrogen in the presence of a catalyst. Hydrogenation catalysts which are said to be able to be used are metals such as Pt, Pd, Au, Ag, Zn, V, W, Co, Ni, Ru, Rh, Mn, Cr, Mo, Ir, Os, Pb, their alloys and also their oxides and sulfides. As preferred hydrogenation catalysts, mention is made of pyrophoric metal hydrogenation catalysts comprising nickel, cobalt and iron. These hydrogenation catalysts, which are known as Raney catalysts, can be dispersed as finely divided powders in the reaction solution or can also be used as fixed-bed catalysts in the form of relatively large particles. A further possibility mentioned is to apply the metal catalyst to a support material such as pumice or kieselguhr. Disadvantages of the finely divided Raney catalysts are their pyrophoric properties, which mean that the powders have to be handled under inert gas, and also their toxicity. Reactions using finely divided Raney catalysts are predominantly carried out batchwise, and the suspension has to be filtered after the reaction. Carrying out these reactions continuously is relatively difficult, since continuously the reaction mixture has to be filtered and the catalyst retained on the filter has to be returned to the reactor. Furthermore, the production of Raney catalysts, which are suitable as fixed-bed catalysts, is technically complicated.
It is an object of the present invention to provide a single-stage process for preparing pentanediols from alkoxydihydropyrans, which makes do without the use of Raney catalysts.
We have found that this object is achieved by a process for preparing 1,5-pentanediols of the formula (I) 
by single-stage reaction of alkoxydihydropyrans of the formula (II) 
where, in the formulae (I) and (II),
R, Rxe2x80x2, Rxe2x80x3, Rxe2x80x2xe2x80x3 can be identical or different and are each hydrogen or a linear or branched saturated hydrocarbon radical having from 1 to 20 carbon atoms in which the hydrocarbon chain may contain O, S and N as heteroatoms and which may be monosubstituted or polysubstituted by hydroxy, thiol or amino groups or halogens,
with water and hydrogen in the presence of a catalyst comprising oxides of nickel, zirconium and copper.
The catalyst used according to the present invention comprises oxides of nickel, zirconium and copper. The catalyst may further comprise molybdenum oxide. The catalyst used according to the present invention preferably comprises from 20 to 75% by weight, particularly preferably from 30 to 70% by weight, very particularly preferably from 40 to 60% by weight and especially from 35 to 55% by weight, of nickel oxide, preferably from 10 to 75% by weight, particularly preferably from 10 to 60% by weight, very particularly preferably from 15 to 50% by weight and especially from 25 to 45% by weight, of zirconium dioxide and preferably from 5 to 50% by weight, particularly preferably from 5 to 40% by weight, very particularly preferably from 10 to 35% by weight and especially from 10 to 20% by weight, of copper oxide. The catalyst may further comprise up to 5% by weight, for example from 0.1 to 5% by weight, of molybdenum oxide. The proportions by weight indicated are in each case based on the oxidic, unreduced catalyst and add up to 100% by weight.
In one embodiment, the catalyst used according to the present invention comprises nickel oxide, zirconium dioxide and copper oxide and no molybdenum oxide. In a further embodiment, the catalyst used according to the present invention further comprises 0.1-5% by weight of molybdenum oxide. The catalysts according to the present invention preferably comprise only the metals nickel, zirconium, copper and, if desired, molybdenum and any further metals only in traces, for example in amounts of  less than 1 mol %, preferably  less than 0.1 mol %, based on the total metal content. Preference is thus given to catalysts which consist essentially of the abovementioned metal oxides in the amounts specified above.
In general, the catalysts used according to the present invention are used in the form of unsupported catalysts. For the purposes of the present invention, the term xe2x80x9cunsupported catalystxe2x80x9d refers to a catalyst which, in contrast to a supported catalyst, consists only of catalytically active composition. Unsupported catalysts can be used by introducing the catalytically active composition milled to a powder into the reaction vessel or by converting the catalytically active composition into shaped catalyst bodies, for example spheres, cylinders, rings or spirals, by milling, mixing with shaping aids, shaping and heat treatment and installing these in the reactor.
In general, precipitation methods are employed for preparing the catalysts used according to the present invention. Thus, for example, they can be obtained by coprecipitation of the nickel and copper components from an aqueous salt solution containing these elements by means of mineral bases in the presence of a slurry of a sparingly soluble, oxygen-containing zirconium compound and subsequent washing, drying and calcination of the precipitate obtained. Sparingly soluble, oxygen-containing zirconium compounds which can be used are, for example, zirconium dioxide and hydrated zirconium oxide. Molybdenum can be added before drying as ammonium heptamolybdate.
The catalysts used according to the present invention can be obtained by coprecipitation of the nickel and copper components by adding an aqueous mineral base, in particular an alkali metal base such as sodium carbonate, sodium hydroxide, potassium carbonate or potassium hydroxide, while stirring to a hot aqueous salt solution containing copper and nickel until precipitation is complete. The precipitates obtained in these precipitation reactions are generally chemically nonuniform and comprise, inter alia, mixtures of oxides, hydrated oxides, hydroxides, carbonates and insoluble basic salts of the metals used. It may be found to be advantageous to age the precipitates to improve their filterability.
The catalyst used according to the present invention is preferably prepared by precipitating salts of the metals nickel, copper and zirconium in aqueous solution at from 30 to 90xc2x0 C. and a pH of from 5 to 9, filtering the suspension, drying the filter cake and heating it at from 300 to 700xc2x0 C. If desired, molybdenum is added as ammonium heptamolybdate before drying. The precipitation is carried out by mixing an aqueous solution of salts, e.g. the nitrates, sulfates or acetates, of the metals nickel, copper and zirconium, with an aqueous solution of alkali metal carbonate. The amounts of the metal salts are calculated so that the catalyst composition has the specified composition after heat treatment.
In a further, preferred variant of the preparation of the catalyst used according to the present invention, part of the water-soluble zirconium salt, for example a proportion of up to 50% by weight based on the zirconium used, is replaced by solid zirconium dioxide which is added to the aqueous metal salt solution prior to precipitation or is placed in the reaction vessel.
In the preparation of the catalyst used according to the present invention, the aqueous solution of the metal salts is, for example, mixed simultaneously while stirring with an aqueous alkali metal carbonate solution, preferably sodium carbonate solution, resulting in precipitation of the metals in the form of a mixture of metal hydroxides and metal carbonates. The metal salt content of the metal salt solution is preferably from 30 to 40% by weight. The aqueous alkali metal carbonate solution preferably has a concentration of from 15 to 20% by weight.
The suspension obtained is filtered and washed with water until no more anions can be detected. It is subsequently dried at from 120 to 200xc2x0 C. in a drying oven or in a spray dryer. The molybdenum is, if used, added as ammonium heptamolybdate to the moist filter cake. The dried filter cake is heat treated at from 350 to 700xc2x0 C., preferably from 400 to 600xc2x0 C. The catalyst composition obtained in this way can be tableted or extruded prior to use. For example, the catalyst composition is mixed with a tableting aid, preferably graphite, and pressed to give pellets having dimensions of 6xc3x973 mm. The pellets produced in this way are heat treated at from 300 to 700xc2x0 C., preferably from 400 to 600xc2x0 C. The pellets obtained in this way generally have a mean density of from 1500 to 1900 g/l, a porosity (determined by water absorption) of from 0.2 to 0.4 ml/g and a hardness of from 3000 to 4000 N/cm2. The catalyst obtainable in this way is generally subjected before use in the process of the present invention to a reductive treatment with hydrogen at from 200 to 350xc2x0 C., preferably from 230 to 280xc2x0 C., for a period of, for example, from 20 to 40 hours at a hydrogen pressure of generally from 1 to 300 bar, preferably from 100 to 150 bar.
The catalysts used according to the present invention can also be prepared by peptizing pulverulent mixtures of hydroxides, carbonates, oxides and/or salts of nickel, zirconium, copper and, if desired, molybdenum with water and subsequently extruding and heat treating the compositions obtained in this way.
The 1,5-pentanediols of the formula (I) are obtained by reaction of alkoxydihydropyrans of the formula (II) according to the reaction equation below: 
In the formulae (I) and (II), R, Rxe2x80x2, Rxe2x80x3, Rxe2x80x2xe2x80x3 can be identical or different and are each hydrogen or a linear or branched saturated hydrocarbon radical. Preferred hydrocarbon radicals are C1-C8-alkyl such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, sec-pentyl, neopentyl, 1,2-dimethylpropyl, n-hexyl, isohexyl, sec-hexyl, n-heptyl, isoheptyl, n-octyl, isooctyl and 2-ethylhexyl; the C1-C4-alkyl radicals mentioned are particularly preferred.
In the hydrocarbon radicals, O, S and N may be present as heteroatoms in the hydrocarbon chain. Examples of such radicals are C2-C20-alkoxyalkyl, preferably C2-C8-alkoxyalkyl, particularly preferably C2-C4-alkoxy-alkyl such as methoxymethyl, ethoxymethyl, n-propoxymethyl, isopropoxymethyl, n-butoxymethyl, isobutoxymethyl, sec-butoxymethyl, tert-butoxymethyl, 1-methoxyethyl and 2-methoxyethyl, C2-C20-alkylthioalkyl such as the alkylthioalkyl radicals corresponding to the abovementioned alkoxyalkyl radicals, C3-C20-dialkyl-aminoalkyl, preferably C3-C10-dialkyl-aminoalkyl such as dimethylamino-methyl, diethylaminoethyl, di-n-propylaminoethyl and diiso-propylaminoethyl, and C2-C20-alkylaminoalkyl, preferably C2-C8-alkylaminoalkyl such as methylamino-methyl, methylaminoethyl, ethylaminomethyl, ethylaminoethyl and isopropyl-aminoethyl.
The hydrocarbon radicals mentioned can be monosubstituted or polysubstituted by hydroxy, thio or amino groups or halogens.
Particular preference is given to using alkoxydihydropyrans of the formula (IIa) 
where R is linear or branched C1-C4-alkyl.
In particular, use is made of the following alkoxydihydropyrans:
2-methoxy-2,3-dihydro-4H-pyran, 2-ethoxy-2,3-dihydro-4H-pyran, 2-propoxy-2,3-dihydro-4H-pyran, 2-butoxy-2,3-dihydro-4H-pyran and 2-isobutoxy-2,3-dihydro-4H-pyran.
The process of the present invention is preferably carried out at from 50 to 250xc2x0 C., particularly preferably from 100 to 200xc2x0 C., and a pressure of preferably from 100 to 300 bar absolute, particularly preferably from 50 to 220 bar absolute. The pressure is maintained by addition of hydrogen. The process can be carried out continuously or batchwise and in both cases hydrogen can be circulated. The reactor can be operated in suspension or fixed-bed mode. Water and the alkoxydihydropyran are introduced in liquid form into the reactor, preferably in a molar ratio of water to alkoxydihydropyran of from 1.5:1 to 10:1, particularly preferably from 3:1 to 8:1, based on the freshly introduced feed. In general, from 0.1 to 5.0 kg, preferably from 0.3 to 1.5 kg, of alkoxydihydropyran is reacted per kilogram of catalyst per hour.
The process of the present invention is preferably carried out continuously, and the reactor is preferably operated in the fixed-bed mode. The reactor can be operated either in the upflow mode or in the downflow mode, i.e. the liquid reactor feed can be passed through the catalyst bed either from the top downward or from the bottom upward. The hydrogen can be passed through the reactor either in cocurrent or in countercurrent to the liquid feed.
In steady-state operation of the reactor in a continuous process, part of the liquid reactor product is preferably recirculated to the reactor, with particular preference being given to recirculating from 2 to 10 kg of the liquid reactor product to the reactor for every kilogram of fresh alkoxydihydropyran fed in.
The reactor is preferably followed by a separator in which the reactor product is depressurized and hydrogen gas and liquid are separated from one another. In general, the major part of the hydrogen, preferably from 70 to 95% by volume is recirculated to the reactor. The liquid reactor product taken off can be worked up in a manner known per se. It is preferably subjected to a two-stage rectification in which water and the alcohol formed, for example methanol, are separated off in the first stage and pure 1,5-pentanediol is isolated in the second stage under reduced pressure.